1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating, acquiring and processing seismic data and, more particularly, to mechanisms and techniques for separating seismic data simultaneously shot by two or more seismic sources that may or may not belong to the same seismic survey. The separation process is referred to as deblending.
2. Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of geophysical structures under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.
In the past, conventional land seismic acquisition generally employed multiple vibrators (seismic sources) acting one at a time. The vibrators are positioned at a source location and then actuated. Once activated, the vibrators generate a sweep that typically lasts between five and forty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. For reflection seismology, the record length may be considered as the sweep length plus a listening time equal to the two-way travel time of the deepest structure of interest, which is the time required for the seismic energy to propagate from the source through the earth to the deepest reflector of interest and back to the receiver. The vibrators are then moved to a new source location and the process is repeated.
For marine acquisition, a seismic acquisition system 100 includes, as illustrated in FIG. 1, a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source array 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to the head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to the tail end 1108 of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth. Seismic sensors 122 are distributed along the streamer and configured to record seismic data. Seismic sensors 122 may include a hydrophone, geophone, accelerometer or a combination thereof. Positioning devices 128 (also known as birds) are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which are typically air guns. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. The source elements attached to float 137 form a sub-array. Source array 130 may have multiple sub-arrays, typically three. Traditionally, vessel 102 tows two source arrays 130 and 130′, which may be identical. During operation, vessel 102 follows a predetermined path T while source elements (usually air guns) 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144, which are recorded by sensors 122. The positions of both source elements 136 and recording sensors 122 may be estimated based on GPS systems 124 and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even fully process the data. Controller 126 may also be connected to the vessel's navigation system and other elements of the seismic survey system, e.g., positioning devices 128. The above configuration may be modified to replace the streamers with ocean-bottom receivers in an ocean bottom survey (OBS). These may be ocean bottom nodes (OBN) or ocean bottom cables (OBC). In this case, one or more vessels tow only seismic source arrays while the seismic receivers, which are part of the OBS, are stationary on the ocean bottom. The seismic receivers record the seismic signals and store them on a local memory. In the OBN case, the recorded seismic data is then retrieved from each OBN and processed as discussed later.
When the source (either land source or marine source) is fired with standard data acquisition, the subsequent recording time is defined so that all useful reflected/diffracted energy is recorded before the next shot fires. This delay time imposes constraints on the acquisition rate and, hence, increases the cost and length of the acquisition process.
To reduce acquisition time, it is possible to simultaneously shoot sources. Acquisition of simultaneous source data means that the signals from two or more sources interfere at least for part of the record; one source is shot during the listening time of another source so that a same seismic receiver receives during that listening time information corresponding to both sources. By acquiring data in this way, a process known in the field as simultaneous source acquisition, the time taken to shoot a data set and acquisition costs are reduced. As an alternative to reducing acquisition time, a higher density data set may be acquired at the same time. For such data to be useful, it is necessary to develop processing algorithms to handle source interference (cross-talk noise). The separation of energy from distinct seismic sources is referred to here as a deblending process.
Simultaneous source acquisition can be performed in land, transition, and marine environments (e.g., Ocean Bottom Node (OBN), Ocean Bottom Cable (OBC), towed streamers, autonomous underwater vehicles, etc.), with continuous or non-continuous recording. FIG. 2A shows traces recorded in time for various shot point (SP) locations, where the successive firing time of SP excitations is equal to or larger than a listening time. FIG. 2B illustrates the concept of continuous recording for simultaneous shooting acquisition, i.e., reflected signals from SP1 are recorded during the listening time for SP2.
The most common methodology for using the simultaneous data of FIG. 2B is to separate the energy associated with each source (to deblend) as a preprocessing step, and then to proceed with conventional processing steps.
To make the separation easier, it is generally advantageous to use a variety of different source signals, for example, different vibroseis sweeps or pseudo-random sweeps. When energy from a given source is time-aligned, this also allows a designature operator to be applied that focuses the energy of that source while keeping energy from other sources at least partially dispersed. The designature operator may be reversed (e.g., convolving by the source signature) in the event energy is reverse time-aligned. The process of designature or resignature may optionally be included whenever time alignments or time shifts are used.
With impulsive source acquisition, e.g. airgun marine acquisition, it is common to desynchronize timing between the two sources such that when data from one source is time-aligned (see data 202 in FIG. 2B), energy 204 from other sources appears with an irregular timing.
The deblending process is closely related to seismic interference noise attenuation. Therefore, methods such as those described in Lynn et al., 1987, Haldorsen and Farmer 1989, and Huaien et al., 1989 (see the end of the specification for more details about these references), for example, can be used to remove blending noise from the data. The method described in Huaien et al., (1989) closely resembles methods specifically developed to deblend simultaneous shooting data by sorting the data into a certain domain, applying a coherency enhancing filtering process, and sorting the data back to the original domain. In the literature, methods specifically developed for the deblending of simultaneous shooting data generally fall into one of the categories now discussed.
One deblending approach is known as separation in a model domain, which is illustrated by Trad et al., 2012. This paper proposed a method that separates signal and cross-talk by muting in the apex-shifted Radon domain. Another deblending approach is impulsive denoising. This denoising method (see, e.g., Stefani et al., 2007) uses the fact that when data is sorted into any domain other than common shot, the cross-talk noise from other sources has a random timing, as illustrated in FIG. 3 from Hampson et al., 2008. This random timing allows the use of impulsive noise attenuation techniques, which are already available and used in other processing steps, such as, for example, swell noise attenuation. Still another approach is iterative coherency enhancement/denoising. Iterative signal enhancement/denoising techniques (e.g., Vaage, 2003; Abma and Yan, 2009; Maraschini et al., 2012; Maraschini et al 2012b; Mandad, et al., 2012; and Peng et al., 2013) rely on the fact that cross-talk noise on some traces is always a duplication of signal energy on other traces. A signal estimate for one source is used to reduce the level of cross-talk for other sources. Another approach is simultaneous modeling of energy from all sources. This modeling scheme (e.g., Akerberg et al., 2008; and Moore et al., 2008) solves the relationship between source energy and cross-talk noise by designing a transform domain for each source or spatial area (e.g., tau-p domain, Fourier domain, etc.) such that when it is reverse-transformed and re-blended, the raw input data is reconstructed accurately in a least-squares sense. Once the transform domain has been calculated, the final step to deblend the data requires application of reverse transform without re-blending.
However, existing methods inherently damage the signal during the process of removing cross-talk. Thus, there is a need to develop a method capable of processing blended seismic data while not being limited as noted above.